Cellular behavior is controlled by environmental signals including nutrients, osmotic stress, hormones and neurotransmitters. Only recently have investigators begun to address how the response to any of these stimuli varies from cell to cell. This project focuses on mechanisms of noise regulation in cellular signal transduction and their role in determining cell fate. Our investigation exploits a prototype G protein signaling pathway in yeast. In this system, a peptide mating pheromone activates a cell surface receptor, a G protein, and a MAP kinase signaling cascade. Our recent work revealed that several pathway components act in a dynamic manner to regulate the time-dependent noise characteristics of the pathway. This observation motivated our primary hypothesis that the sources of noise responsible for cell-to-cell variability are regulated to promote cellular survival under changing environmental conditions. In certain contexts, fluctuations may serve as bet-hedging mechanisms to diversify the response of a population of isogenic cells, whereas in other contexts noise suppression may be required to properly coordinate response pathways when cells are faced with multiple competition stimuli. Our research plan uses microfluidic devices and fluorescent imaging to follow single cells in well controlled environments, quantitative image analysis to characterize fluctuations in signaling and gene expression, and stochastic modeling to suggest and test noise regulation mechanisms. The following aims will be used to determine the role of noise in shaping cellular behavior and identify the biological circuits that regulate is properties: Aim 1 analyzes effects of noise on the cellular decision to differentiate or proliferat. Aim 2 analyzes mechanisms of noise suppression required for coordination between two MAP kinase pathways, one that promotes differentiation and one that is required for adaptation to osmotic stress. Aim 3 analyzes effects of noise in the transition to age-dependent sterility. Our findings will reveal the organization and logic of signaling circuits that regulate noise and to wht extent noise influences cell fate decisions and survival. Such systems level principles are likely to be shared by many signal pathways in organisms ranging from yeast to humans. Yeast is an ideal platform for these analyses; all of the cells in the population are genetically identical, an can be maintained under uniform and easily modulated growth conditions. Genetic manipulability is unparalleled and the consequences of any changes are easily quantified. Thus it is practical to modulate noise by genetic or environmental changes, measure the functional consequences, develop computational models and test them experimentally. This integrated approach is ideal for both hypothesis-generating and hypothesis- testing. Finally, the signaling pathway in yeast employs cell-surface receptors, G proteins, MAP kinases, and cyclin dependent kinases (CDKs) homologous to those found in humans. Thus signaling mechanisms identified in yeast will inform human physiology and pharmacology.